Process for improving the wettability of solid metallic surfaces by molten alkali metals



United States Patent Oice 9 Claims. (61. 117-210 This invention relates to a process for improving the wettability of solid metallic surfaces by molten alkali metals, particularly in the form of sodium-coated electrodes.

A process permitting the cathodic deposition of sodium under very mild conditions with the use of complex organic compounds of aluminum and boron as electrolytes is described in patent application 27,219 filed May 5, 1960 abandoned in favor of 299,689, filed July 31, 1963, now Patent 3,234,115 and 192,954, filed May 7, 1962, now Patent 3,234,113. This process is particularly interesting in connection with the recovery of metallic sodium from sodium amalgam obtained, for example, in the conventional alkali chloride electrolysis in an aqueous solution or in the production of metal alkyl compounds by electrolysis of aluminumor boron organic electrolytes as described, for'example, in US. patent applications 27,220, filed May 5, 1960, abandoned in favor of 129,009, filed Aug. 3, 1961 now Patent 3,164,538, 114,939 filed June 5, 1961, abandoned in favor of 320,607, filed Oct. 29, 1963, now Patent 3,285,947, and 129,009 filed Aug. 3, 1961, now Patent 3,164,538.

The process is also'useful forthe recovery of a highly pure sodium which, for example, is recently used as heat transfer medium for nuclear reactors.

When-carrying out this electrolysis for the cathodic recovery of sodium, the latter, particularly at temperatures slightly above its meltin point, is separated in electrolytes which have a density very close to that of the molten sodium. However, the density of conventional electrolytes is slightly less than that of the molten metal. For this reason, the molten sodium in the electrolytic cells has the tendency to sink downwardly, which may result in troubles in case of certain conventional cell constructions. For example, when operating the cell with a sodium amalgam anode using a thin layer of horizontally flowing sodium amalgam as the anode, there exists the possibility that the molten sodium metal deposits in droplets at a cathode arranged horizontally above the flowing amalgam, which droplets become detached after some time and drop back into the amalgam. This results in a considerable reduction in the current yield and in troubles in the cell because the re-combination of the sodium with the mercury is accompanied by evolution of considerable heat and, moreover, solid amalgam particles may form since the concentration of the sodium at those points where the droplets fall into the mercury is very much higher than that corresponding to the liquid amalgam.

Certain means which may be used to overcome the difliculties mentioned above are disclosed in US. patent application 27,219 which suggests to cover the cathode with a thin, fine-meshed net of insulating material through which sodium cannot penetrate due to its surface tension.

When using vertical cathodes of the type which might be combined with a rotary disc anode for sodium amalgam, a known phenomenon is that if the cathode is made of a suitable material such as copper, the sodium is eventually deposited in the form of a continuous film which runs down the cathode and, by appropriately bending the cathode, may be withdrawn at a suitable point at the bottom of the cell.

Patented Nov. 5, 1968 When cells provided with cathodes of this kind are started up, the following difliculty has been encountered: While it can be expected, especially when bare copper surfaces are used as the cathode material, that a continuous sodium film of the kind described above will be formed eventually, this will not occur as a rule and not with suificient dependability. It will happen frequently that at least initially the sodium will combine in discrete droplets which remain attached to the electrode surface. These spheres will then preferably continue to grow because their surfaces are at a smaller distance to the anode than the cathode surface proper. It is obvious that this may eventually lead to troubles, particularly to short circuits. When combining such a methal cathode with the net mentioned above, a substantial improvement of'the phenomena described may be obtained. However, even then troubles may occur occasionally if the metal breaks through due to defects in the net which may occur from time to time.

The invention relates to a development which provides the possibility to overcome the difliculties described above and by means of which it is generally possible to make metal surfaces readily and reliably wettable by molten alkali metals. The invention is particularly suitable for the preparation of sodium-coated electrodes which operate reliably from the beginning of the electrolysis. In particular, cathodes used for the deposition of molten sodium from the above-mentioned electrolytes can be treated such that they operate completely satisfactorily and free from troubles.

It has been found that completely reliable wettability of metal surfaces by molten alkali metal, particularly sodium, can be achieved by providing the surfaces with a thin coating of a metal which is particularly suited for this purpose and which is preferably readily dissolved in the deposited molten alkali metal. The best result is shown by cadmium, but gold may also be used besides cadmium. Best results are obtained in both cases with copper as the substrate. However, especially when using cadmium, excellent results have also been obtained on other metals such as iron, special steel, aluminum, nickel. Doubtlessly outstanding results are achieved with cadmium-plated or gold-plated copper cathodes. In case of other base materials such as iron, it is preferable but not mandatory to apply a copper coating prior to cadmium-plating. The coating metals cadmium or gold remain by no means attached to the cathode during the electrolysis. They are rather dissolved very rapidly in the first portions of sodium deposited electrolytically. The metal surface beneath the cadmium plating will remain completely wettable even after the plating has been dissolved and the cathodes will perform fully satisfactorily as long as the coating of the metal surface with the alkali metal film will not be interrupted and the electrolytic cell dismantled, e.g. for repair work. After an interruption cathodes of this kind must again be provided with the cadmium or gold plating. The coating metal migrates out of the cells very rapidly. As early as a few hours after the start-up, the cell will furnish a sodium which is completely free from cadmium or gold. The content of said metals is not high even during the first hours of operation because very thin coatings are absolutely suificient to bring about the effect of the invention, coatings of 1 to 5 microns thickness being fully adequate. Of course, thicker platings are not detrimental but unnecessary. The platings are easily applied in a manner known per se, e.g. by electrolytic deposition.

The object of the invention, i.e. reliable wettability of the electrode throughout the performance of the electrolysis and avoidance of troubles in the electrolysis by irregular flow of the liquid metal on the electrode and especially irregular deposition of metal at the electrode or outflow of the deposited metal in'an undesirable direction, is further developed in accordance with'the invention, additionally tothe embodiment described above, by another fundamental idea. This further development which is independently inventive besides the embodiment described aboveresides in the teaching to enlarge the inner surface area of the electrode material. Any specific electrode form which leads to such an enlarged inner surface area falls'under the invention; Outstanding results are obtained especially in those cases where the characteristics of improved wettability of the cathode material by plating with an auxiliary metal as described are combined with the characteristic of enlarged inner surface areas of the cathode material.

The essential features of this enlarged. inner surface area is the fact that electrodes which are structurally loosened up are used as the electrode material in place of' a metal plate. The enlargement of the inner surface area of the electrode may be achieved by various methods. Thus, a considerable improvement of the effectiveness of the cathodes is obtained if, e.g. in case of cathodes of metal plate, the latter is perforated in a most simple manner. If as high as possible a number of perforations is used, the particularly efficient cathode of the invention consists of a plate provided with many holes and coated with cadmium or gold, if desired after copper plating, said coating being preferably applied to both sides. Such a cathode, even when involved in the electrolysis on one side only, becomes completely covered by molten sodium which, by adhesion and enlargement of the area of contact between the base metal and sodium, is fixed very well to the plate. Adhesion to the base metal will still be improved by arranging a second identical or unperforated plate at a distance of a few'millimeters from the first plate and parallel with respect to the latter so that the intermediate space may also become filled with sodium. When limiting the possible layer depth of the sodium towards the anode by insertion of a net, e.g. a glass cloth, then an arrangement has been provided from which the sodium' will substantially never flow out and from which themolten sodium -'may be withdrawn continuously in a suitable manner toward the side or in upward or downward direction.

While the electrodes of the invention have been developed primarily as cathodes, they may also be used as anodes (for raw sodium to be refined) if care is taken by constant supply of liquid sodium constantly filled.

What is claimed is: 1. Process of Wetting a solid metallic surface inert to molten alkali metal with alkali metal which comprises: (a) coating the metal with a film of cadmium or (b) contacting the film while on'said surface with molten alkali metal;

(c) allowing molten alkali metal and the metal of 'said coating to combine and removing the combined molten alkali metal and metal of-said coating 'leaving molten metallic alkali covering and'wetting said surface.- 1

'2. Process according to claim 1, wherein said-"cad-- mium or gold coating is applied in a thickness of about 1 to'5' microns.

3. Process according metal is sodium.

4. Process according to claim 1,

minum or nickel.

5. Process according to claim 1, wherein said metallic surface is copper-plated base metal.

6. Process according to claim 1, wherein structurally expanded metal cathodes having an enlarged inner surface are provided with the coating of cadmium or gold.

7. Process according to claim 1, wherein the solid metallic surface is the surface ofa perforated metal plate and the surface is coated with cadmium or gold.

8. Process according to claim 1, wherein thesOlid metallic surface is the surface of a metal sponge electrode and the surface is coated with cadmium'or gold.

9. Process according to claim 1, wherein the solid metallic surface is the surface of a wire net electrode and the surface is coated-with cadmium or gold.

References Cit ed L.- JARVIS, Primary Examiner.

that they remain to claim 1, Wherein saidalkali wherein said metallic. surface inert to molten alkali is copper, iron, steel, alu- 

1. PROCESS OF WETTING A SOLID METALLIC SURFACE INERT TO MOLTEN ALKALI METAL WITH ALKALI MTAL WHICH COMPRISE: (A) COATING THE METAL WITH A FILM OF CADMIUM OR GOLD; (B) CONTACTING THE FILM WHILE ON SAID SURFACE WITH MOLTEN ALKALI METAL; (C) ALLOWING MOLTEN ALKALI METAL AND THE METAL OF SAID COATING TO COMBINE AND REMOVING THE COMBINED MOLTEN ALKALI METAL AND METAL OF SAID COATING LEAVING MOLTEN METALLIC ALKALI COVERING AND WETTING SAID SURFACE. 